A reader in the U.K., Len Threadgold, (see profile below) commented on last week’s blog. (Ref. 1) Len speaks with some authority when he notes that the problems with water can be grouped under the following three headings. He also notes that separating problems into neat categories is useful but sometimes the categories work in association with each other – see examples below.
This classification is good: A way of understanding natural events and environmental processes. In a sense, a way of measuring things. Engineers like that. It helps us solve problems and explain causes and technical issues to the justice system. As such, it’s a forensic engineering method:
1. The problems that the presence of water causes
We don’t like flooding or working under water - so this problem concerns water level.
Examples would be your flooded basement, also the water pooling in your back garden that has drained from the land above - sorry to bring these examples up. And the recent flooding that occurred in Truro, N.S., and in the U.K., and forecast for the Saint John River, New Brunswick. Water pressure would also be a factor in a wet basement; see #3 below.
2. The problems that the flow of water causes
These problems are about erosion, the removal of soil as a result of water flow, the speed of the water.
For example, coastal erosion such as that occurring around the entire coast line of Prince Edward Island and also at Red Head, on the Bay of Fundy near Saint John, New Brunswick. The flowing water on the coast is wave action and long shore currents.
I investigated one landslide at Red Head some time ago that destroyed a home at the top of a sea cliff. I learned two weeks ago that erosion of the bottom of the cliff continues to remove the buttressing effect of the soil there.
Rain triggered the landslide at Red Head that I investigated – after erosion of the toe set the stage. Rain did this by increasing the pressure in the ground water behind the cliff; see #3 below.
3. The problems that the pressure of water causes
These problems arise because water pressure affects soil strength. You know this when your boots sink into the mud – clay in engineering – after a rain storm.
The problems are bigger than sinking, muddy boots though.
Water pressure – what we call pore water pressure in engineering - is often a factor in landslides as it was for the landslide at Red Head. At times water pressure is the principal cause in changing an adequately stable slope to an unstable one. Its effect is to reduce the frictional strength of the soil.
Water pressure was certain to have been a factor in the recent landslide in Washington State, U.S. This landslide took more than a dozen lives with others still missing. I saw early reports that there was a known risk of a landslide at this location. The assessment leading to that conclusion is certain to have considered pore water pressure and its potential to change.
Len has investigated the potential for landslide problems in Hong Kong. These would be slopes that were susceptible to increases in pore water pressure. Simple drains often fixed the problems but sometimes more in-depth water interception was necessary.
Water pressure is a factor in our wet basements. And, believe or not, in many slip and fall accidents. It’s the reason signs in a swimming pool caution us not to run on the pool deck.
In some slip and fall accidents, a person’s weight is momentarily applied to water on the floor surface. The frictional or skid resistance of water is much lower than the material forming the floor surface – almost negligible by comparison. So low in fact that the person slips and falls.
Len’s classification is a good one, an aid to those of us investigating problems with water and needing to explain the cause of a problem to the justice system.
Many of the forensic engineering problems I investigate - not just a few, and not just the obvious drainage and flooding problems - can be traced back to water and fall under one or more of the headings Len has identified.
1. Image credits and why forensic engineers like wet weather, the heavier the rain the better, posted April 9, 2014 http://www.ericjorden.com/blog/2014/04/09/image-credits-and-why-forensic-engineers-like-wet-weather-the-heavier-the-rain-the-better/
Len Threadgold’s Profile
Len is a civil engineer in the U.K. specializing in soil, rock, and ground engineering – geotechnical engineering. We were colleagues when I practiced there. Len’s firm, Geotechnics Ltd, provides consulting services to an international clientele including dealing with slope stability problems in Hong Kong and the U.K. – problems that would fall under #3 above. The U.K. had their share of severe flooding problems this past year, problems with the mere presence of water; #1 above. Len read a draft of this blog because it’s his classification system and some of the examples and comments are his.